Interpretive Summary: Cotton is the most important textile fiber crop where a fiber is an extension of a single cell in a developing seed by massive levels of cellulose biosynthesis. Cellulose is known to be the most abundant molecule on earth; yet, very little is known about genes, proteins and metabolic pathways which lead to its synthesis in plants. The early work in corn by Agricultural Research Service scientists in Gainesville, FL, and later Dr. Delmer's work on cotton, has shown that the enzyme sucrose synthase (SuSy) plays a critical role in cellulose biosynthesis in these two important food and fiber crops. However, unlike corn, there is no basic knowledge on SuSy genes and their patterns of expression in cotton, especially in a developing seed. The collaborative study described here utilizes the resources and combined knowledge of two diverse groups to fulfill such a deficiency. Overall, the results show that cotton harbors multiple SuSy genes and exhibits a complex pattern of expression among various tissues of the plant. Based on biochemical, cellular and molecular studies, a model is presented to show how SuSy enzyme metabolizes sucrose (synthesized in leaf by photosynthesis) into diverse functions, including cellulose biosynthesis. Clearly, future research on altered quality or quantity of fiber production will have to include modification of SuSy genes in cotton.

Technical Abstract:
The developing cotton seed exhibits complex patterns of carbon allocation in which incoming sucrose is partitioned to three major sinks: the fibers, seed coat and cotyledons; synthesizing cellulose, starch, and storage proteins;and oils respectively. This study investigated the role of sucrose synthase (SuSy) in the mobilization of sucrose into such sinks. Assessments of SuSy gene expression at various levels lead to the surprising conclusion that, in contrast to other plants, SuSy does not appear to play a role in starch synthesis in the cotton seed. However, our demonstration of functional symplastic connections between phloem unloading area and the fiber cells as well as the SuSy expression pattern in fibers indicates a major role of SuSy in partitioning carbon to fiber cellulose synthesis. SuSy expression is also high in transfer cells of the seed coat facing the cotyledons. Such high levels of SuSy could contribute to the synthesis of the thickened cell walls and to the energy generation for sucrose efflux to the seed apoplast. The expression of SuSy in cotyledons also suggests its role in protein and lipid synthesis. In sum, the developing cotton seed provides an excellent example of the diverse roles SuSy can play in carbon metabolism.